Synthesis and self-assembly of FePt nanoparticles have been of great interest to the data storage industry as a possible means for making nanoparticle-based magnetic media which can enable recording densities beyond 1 Tbit/in2. The magnetic bits, which record data in conventional thin film magnetic media, are comprised of many grains. In order to achieve high recording densities with bits having well-defined shapes and boundaries, it is necessary to reduce the so-called transition jitter, which is the dominant noise source in today's media. This is usually accomplished by decreasing the size of the grains such that they remain significantly smaller than the bit size. If the grains become too small, however, thermal fluctuations may cause individual grains to switch randomly, thereby destroying the bit information over time. To avoid that problem, hard magnetic materials systems such as L10 phases or high anisotropy rare-earth transition metal systems are being developed, allowing small grains to remain thermally stable. FePt is the most prominent L10 candidate material for this approach. See for example D. Weller and A. Moser, IEEE Transactions on Magnetics, Vol. 35, pp. 4423-4439 (1999). A big limitation of these materials is that typical synthesis methods produce a face centered cubic (fcc), low magnetocrystalline phase and not the desired high anisotropy L10 phase. Thus the films must be subjected to post synthesis annealing to induce a phase transformation from fcc to L10.
Self-assembled arrays of FePt L10 nanoparticles can, in principle, reduce transition jitter by having uniformly sized magnetic grains separated by a well-defined matrix. Jitter is the noise in the magnetic signal from the media that arises from the poorly defined boundary of the magnetic bit formed by randomly shaped and randomly sized magnetic grains. Thus the ideal magnetic media would be formed of nanoparticles that are of uniform, small size, are packed into arrays with the highest possible density, and have uniform, e.g., perpendicular magnetic orientation.
Chemical syntheses have been developed that produce nearly spherical FePt nanoparticles that are highly monodispersed with uniform diameters of 3-4 nm. Various methods of chemical synthesis of FePt nanoparticles are known. These syntheses produce spherical nanoparticles coated with films of oleic acid and oleylamine surfactants that are ˜2 nm thick. One of the disadvantages of these small spherical nanoparticles is that they have low magnetic moment per unit area requiring very sensitive read heads to detect the recorded bits. A typical parameter used to describe the average signal from the media is the Mrt. Here Mr is the remanent magnetization and t is the thickness of the media. FIGS. 7 and 8 show calculations for the Mrt for spherical particles self-assembled on square and hexagonal lattices and cubes self-assembled on a square lattice. Taken into consideration is a region of ‘nonmagnetic’ material surrounding the particles with a thickness of ‘a’, which may represent surfactant, oxides, other nonmagnetic metals and/or space. The Mrt for all these structures are shown to be ˜kMrr3/R2 where Mr is the remanent magnetization and r and R are geometrically described in the figure. Notice that the constant k depends on the shape of the particles and the lattice in which they are placed. Here k=1.05 for spheres on a square lattice, k=1.21 for spheres on a hexagonal lattice, and k=2 for cubes on a square lattice. Clearly self-assembled spherical particles give relatively lower Mrt as compared to cubic particles with similar dimensions.
A more critical problem with spherical particles is the fact that the magnetic axes of the particles are difficult to align once deposited on a surface. Others have reported the fabrication of oriented FePt nanoparticles with high coercivity but these nanoparticles were fabricated by electron beam evaporation and grown epitaxially on MgO(001) and NaCl(001) surfaces. Increasing the packing fraction and obtaining alignment of the magnetic axis are both crucial for utilization of chemically synthesized FePt nanoparticles in high density recording media.
Synthesis of cubic nanoparticles has been reported in the literature for various metals and alloys, including monodispersed silver nanocubes with edge length of 55 nm, cubic PbTe nanoparticles, and cubic PbSe nanoparticles for use as quantum dots. The formation of core shell cubic FePt/Fe3O4 nanoparticles has been reported, but showed no net orientation or long range self-assembly of these nanoparticles when deposited on substrates. Chemical synthesis of cubic FePt nanoparticles was reported by H. Zeng et al. [J. Am. Chem. Soc. 126, 11458 (2004)].
Self-assembled magnetic nanoparticle arrays are described in U.S. Pat. No. 7,041,394 B2, the disclosure of which is hereby incorporated by reference.
It would be desirable to have a magnetic storage media that includes nanoparticles having uniformly oriented magnetic axes and long range self-assembly.